Buoyancy: Upward Force by Fluids

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Author: Janice VanCleave

Buoyancy is the upward force of a fluid on an object placed in it. Archimedes, a Greek mathematician, is given credit for explaining the force, called buoyancy, that supports objects in a fluid. Floating is usually associated with water, but floating can describe any object that is suspended in any other fluid—liquid or gas.

In this project, you will measure the buoyancy on a floating object by measuring the weight of water displaced (pushed aside) by the object. You will measure the difference between the weight of an object in air and the weight of the same object submerged in water, and compare the difference to the buoyancy of the water on the object. You will also investigate the relationship between the density of a fluid and its buoyancy.

Getting Started

Purpose: To measure the weight of water displaced by an object placed in it.


  • scissors
  • empty plastic 2-liter soda bottle
  • one-hole paper punch
  • flexible drinking straw
  • metric measuring cup (250 ml)
  • pitcher
  • tap water
  • 3-ounce (90-ml) paper cup
  • 20 pennies


  1. Cut the top section off the soda bottle. Discard the top and keep the bottom of the bottle.
  2. Use the paper punch to make a hole in the plastic about 2 inches (5 cm) from the rim of the bottle.
  3. Insert the short end of the straw into the hole so that the straw forms a 900 angle. Place the measuring cup under the free end of the straw.
  4. Use the pitcher to pour water into the bottle until it is just above the straw. Water will then flow through the straw and into the measuring cup.
  5. When the water stops flowing into the measuring cup, empty the cup. Replace the empty measuring cup under the straw.
  6. Set the paper cup on the surface of the water in the bottle. Support the paper cup as you add the coins to the paper cup one at a time. Do not let water spill over the rim of the bottle.
  7. Buoyancy Upward Force by Fluids

  8. When the water stops flowing through the straw into the measuring cup, remove the paper cup from the bottle. Record the volume of water in the measuring cup in liters (L)(an SI unit for volume).
  9. Use the following equation to determine the mass of the water in kg:
  10. mwater = Dwater × Vwater


    m = mass measured in kg
    Dwater = density of water, which is 1 kg/L
    Vwater = volume of water displaced in liters (L)


    If the paper cup displaced 15 ml of water, which is equal to 0.015 L, then the mass of the water would be:

    mwater = Dwater × Vwater
      = 1 kg/L × 0.015 L
      = 0.015 kg
  11. Use the following equation to determine the weight of the water in newtons (N).
Fwt =mxg
Fwt = weight of water measured in newton units
m = mass of water in kg
g = acceleration of gravity, which is 9.8 m/sec2


If 0.015 L of water is displaced, the mass of the water is 0.015 kg, and the weight of the water in newtons is:

Fwt =m×g
  = 0.015 kg × 9.8 m/ sec2
  = 0.147 kg·m/sec2
  = 0.147 N

Note: 1 newton = 1 kg·m/sec2, so the weight of the water is 0.147 N.

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